Method of reducing slot width in slotted tubular liners

ABSTRACT

A method of reducing slot width in slotted tubular liners. A slotted tubular liner ( 1 ) is provided having an interior surface ( 3 ), an exterior surface ( 2 ) and a plurality of slots ( 4 ) extending between the interior surface and the exterior surface. One or more contoured rigid forming tools ( 7 ) are provided. Pressure is applied to either the interior surface ( 3 ) or the exterior surface ( 2 ) of the slotted tubular liner ( 1 ) with the contoured rigid forming tools ( 7 ). The contoured rigid forming tools are then moved in a sweep pattern traversing either the interior surface or the exterior surface of the slotted tubular liner, until plastic deformation narrows the width of the plurality of slots ( 4 ) to within desired tolerances. The method does not require the same precise positioning of previously known methods and, as such, provides a combination of increased output and lower cost.

This application is a 35 USC 371 of PCT/CA01/01489 filed Oct. 23, 2001.

FIELD OF THE INVENTION

Metal tubulars having through-wall slots are commonly used to line boreholes in porous earth materials to exclude entry of solid particleswhile permitting fluid flow through the tubular wall. The presentinvention provides a method to form the edges of such slots tosubstantially reduce the slot width and preferentially form the shape ofthe through-wall flow channel.

BACKGROUND OF THE INVENTION

Technological advances in directional drilling within the oil industryhave enabled wells to be completed with long horizontal sections incontact with the reservoir. Such long horizontal well bores, often inexcess of 1,000 m, permit fluids to be injected into or produced from amuch greater portion of the reservoir, than would be possible from avertical well, with commensurately greater recovery of petroleum from asingle well. The greater petroleum recovery possible from such wells,more than justifies the increased cost of drilling and completing thehorizontal well section. Additionally, horizontal wells require fewerwellheads with less surface disturbance to exploit the same reserves,providing a collateral environmental benefit. These reasons are strongmotivators to ensure technically and economically viable products areavailable to complete these wells.

For such reservoirs the horizontal section is often completed withslotted steel tubulars (referred to as slotted liners) to preventclosure of the hole through collapse and to function as a screen orfilter permitting flow of injected or produced fluids across the tubularwall while excluding solids. The present invention was conceived as ameans to improve both the technical and commercial viability of slottedliners, particularly needed where the reservoir material is comprised ofweak fine-grained materials.

To function effectively as a filter and structural support member infine-grained reservoirs, and to be sufficiently rugged to endureinstallation handling loads, the slotted liner design is driven by threesomewhat competing needs. To ensure adequate solid particle exclusion,the slot width must be on the order of the smaller sand grain size. Thisis generally true even where fluids are injected, because the effectiveradial stress in the sand tends to force sand grains into the well bore,even though the fluid flows out. For reservoirs comprised of veryfine-grained material, slots less than 0.15 mm may be required. Butsmall slot widths tends to increase flow loss, therefore a greaternumber of slots are needed per unit of contacted reservoir area tomaintain flow capacity, while the greater number of slots must beaccommodated without undue loss of structural capacity. The industryalso recognises advantages for production applications, if the slot hasa ‘keystone’ shape, i.e., the flow channel through the tubular walldiverges from the external entry to internal exit point. This geometryreduces the tendency for sand grains to lodge or bridge in the slot,causing it to plug and restrict flow.

As pointed out by Hruschak in U.S. Pat. No. 6,112,570, the methodsusually used to cut slots through the wall of steel tubulars having awall thickness great enough to provide adequate structural support inhorizontal wells, are not readily applicable for widths less than 0.4mm. Hruschak then goes on to disclose a method where this limitation isovercome by deforming or forming one or both of the external edges of alongitudinal slot, placed in the wall of a steel tubular, to narrow theslot width along its exterior opening. This method relies on applyingpressure along at least one of the longitudinal edges, preferably bymeans of a roller, where such pressure is sufficient to cause localplastic deformation of the metal, and thus permanently narrow the slotto a desired width. As recognized by Hruschak and others using similarmethods, such as Steps in U.S. Pat. No. 1,207,808, this method offorming the exterior longitudinal edges of a slot, has the addedadvantage of producing a ‘keystone’ slot shape where the through-wallchannel shape diverges from the exterior to interior edges of the slot.Processes employing such methods to narrow the slot width by applyingpressure at or along a slot edge to plastically deform it inward arereferred to as seaming.

It will be apparent to one skilled in the art that methods of reducingthe slot width by the application of pressure along or parallel to theedge of a slot, as described by Steps or Hruschak, will be sensitive tothe location where pressure is applied. Specifically, the amount bywhich the slot width is decreased depends strongly on the distancebetween two parallel lines, one coinciding with the slot centre and thesecond with the longitudinal force centre of the pressure applied alongthe slot length. The alignment tolerance may thus be defined as theallowable range of distance between these two lines to meet the requiredtolerance in final slot width. The required tolerance in slot width istypically in the order of +/−0.02 mm. With practical seaming tooling,the associated alignment requirements can be in the order of +/−0.1 mm.

Hence such methods require relatively accurate alignment of the loadapplication means, such as a forming roller, with respect to thecircumferential position of longitudinal slots. To implement this methodin a mechanised process capable of forming a large number of slots onfull-length tubulars, therefore requires considerable sophistication toco-ordinate the positioning of tooling required to perform therespective cutting and seaming operations if conducted sequentially in asingle machine. Even further sophistication is required if the cuttingoperation is performed independent of the seaming. The capital costassociated with such machinery make it difficult to obtain economicallyviable rates of production on full-length tubulars, particularly so ifthe slotting is conducted independent of the seaming.

However it is particularly attractive to decouple the cutting andseaming operations as this allows seaming to be conducted on tubularsslotted by various independent suppliers, improving the economics ofsupply. In this case, circumferential positioning of the longitudinalseaming tools must account for a degree of randomness in thecircumferential distribution of slots obtained from typical suppliers ofslotted liner that significantly exceeds the allowable alignmenttolerance.

SUMMARY OF THE INVENTION

What is required then, is a method of narrowing the width between theexterior edges of longitudinal slots placed through the wall of metaltubulars, that readily accommodates variations in longitudinal orcircumferential slot placement position and is amenable toimplementation in a mechanised process.

To meet these objectives, the method of the present invention providesat least one rigid contoured forming tool with means to apply a largelyradial load to force it into contact with the inside or outsidecylindrical surface of a slotted metal tubular member, the contactedsurface. The radial load thus applied at a location on the contactedsurface, creates a localized zone of concentrated stress within thetubular material where it is contacted, which stress is sufficientlygreat to cause a significant zone of plastic deformation if the contactlocation is near the edge of a slot. Means are also provided tosimultaneously displace said forming tool or tools with respect to thetubular along path lines comprising a sweep pattern on the surface ofthe tubular. The sweep pattern is arranged so that the extended zone ofplastic deformation created as the forming tool passes each point on thepath-line covers an area sufficient to intersect the edges of all slotsto be formed. The method thus consists of ensuring the paths followed bythe displacement of the forming tool or tools while conducting saidsweep pattern, traverse the edges of the slots at a sufficient number oflocations and a sufficient number of times while maintaining sufficientcontact force to plastically form the edges of any slots intersectedalong their entire length. The plastic deformation or forming thuscaused at the edges of the slots tends to narrow the width betweenopposing slot edges along its opening in the contacted surface of theslotted metal tubular. Otherwise stated, the method requires that thearea swept by said extended zone of localized plastic flow, as one ormore rigid contoured forming tools are caused to move over the inside oroutside surface of the slotted metal tubular member, be sufficient tomore than completely cover the edges of all slots to be narrowed byplastic deformation. The swept area need not be continuous over theentire surface of the slotted tubular member but must include the areaof influence from path lines occurring at at least two separatelocations for each slot narrowed.

The primary purpose of the present invention is to employ this method toform the outer edges of largely longitudinally oriented slots placed inthe wall of tubulars suitable for use as liners in wells. The method iscomprised of firstly providing such slotted pipe where the slots,

-   -   extend through the tubular wall providing fluid communication        when in service,    -   have longitudinal peripheral edges,    -   are preferably of approximately equal length,    -   usually have parallel walls,    -   are preferably arranged in rows of circumferentially,        approximately evenly-distributed slots, with rows separated by        short unslotted intervals or rings, effectively forming a        structure where the material between slots act as short beams        joining rings formed by the unslotted intervals, and    -   groups of one or more rows of slots are referred to as a slotted        interval.

Secondly, providing at least one contoured rigid forming tool,preferably in the form of a roller. Thirdly applying pressure to a localarea on the exterior surface of the tubular through the rigid contouredforming tool or tools beginning at one end of a slotted interval.Fourthly, execute a sweep pattern by moving the forming tool or toolswith respect to the pipe to cause it or them to traverse the surface ofthe tubular along a largely helical path a sufficient distance to atleast cover the slotted interval. The contoured forming tool shape, theradial load by which the forming tool is forced against the tubularsurface, the pitch of the helical path and the number of times theoperation is repeated are all adjusted to deform the edges of the slotsalong their length sufficient to continuously narrow each slot to thedesired width.

It will be appreciated by one skilled in the art that the helical sweeppattern employed here is readily able to ‘find’ the edges of all slotsand thus cause them to be formed continuously along their length andthat such helical patterns are commonly used in straightforwardproduction machining operations such as turning or threading. Thisembodiment of the method of the present invention is thus simple tomechanise, readily locates the edges of slots to be formed and may beperformed at high enough surface speeds to readily meet high productionrate requirements. In comparison to the prior art, it therefore enjoysthe benefits of simplified mechanization and therefore reduced capitalcost and higher production rate and is insensitive to variability in thecircumferential position of longitudinal slots.

As recognized by Hruschak, the through-wall channel shape, created bysuch an exterior forming process, is diverging with respect to fluidflow from the exterior to interior of the tubular. This ‘keystone’ shapeprovides the advantage of reduced plugging tendency under inflow orproduction conditions. However if the liner is used in an injectionapplication, fluid flow is from the interior to exterior and the channelshape becomes converging with respect to the fluid flow direction. Wherethe injected fluid contains particulate matter introduced from sourcessuch as the feed stock, mill scale and corrosion products from upstreampiping, or chemical participates, this converging channel shape thustends to encourage plugging and therefore becomes a disadvantage forinjection applications.

An additional purpose of the present invention is therefore to provide amethod to narrow the width of largely longitudinally oriented slotsplaced in the wall of metal tubulars suitable for use as liners in wellsalong their interior edges. To meet this purpose the method of thepresent invention is applied following steps identical to thosedescribed for forming the exterior edges of longitudinal slots exceptthe rigid forming tool or tools are configured to apply pressure to theinterior surface of the slotted tubular. This causes the slot width tobe narrowed along its interior edges creating an inverse keystoneflow-channel shape, which shape is desirable for injection applications.

The geometry of the generally keystone channel shape created by formingthe edges of slots may be further characterized in terms of the rate atwhich the slot width increases with depth from the contacted surfaceedges, i.e., its divergence rate. It will be generally appreciated thatslots with a lesser divergence rate can be expected to plug more easilythan slots with a greater divergence rate for the same reason that thekeystone shape is preferred over parallel wall slots. However if thedivergence rate is very great the formed edges must have less materialsupporting them and are therefore more susceptible to material lossthrough erosion or corrosion. In applications where this material losscauses a significant increase in width the ability to screen to thedesired particle size is compromised.

It is therefore advantageous if the method of forming the slot edges hasthe ability to, not only narrow the slot width, but to control the rateof divergence to more optimally meet the needs of varying applications.The methods of applying pressure along the edges of a longitudinal slotplaced in a tubular work piece to narrow the slot width, as taught byHruschak, partially enable such control but are subject to significantlimitations particularly when mechanized. These limitations may beunderstood by considering how the transverse shape of the forming toolsurface in contact with the tubular, affects the slot divergence rate.This shape may be generally described in terms of its transversecurvature of the forming tool, which may range from convex to concaveand is typically provided as a contoured roller. Hruschak, points outvarious disadvantages of forming the edges of slots with rollers havinga convex radius of curvature, much less than the radius of the pipe, andintended to “bridge” the slot in the manner taught by Steps. Thereforethe more practical range of roller curvature is from slightly concave,through flat to convex. Within this range, it will be evident that aflat or convex roller shape when aligned with the slot and loaded tocause plastic deformation sufficient to narrow the slot to a desiredwidth will tend to plastically flow material over a greater distance oneach side of the slot to a correspondingly greater depth resulting in alesser divergence rate than would be obtained using a more convexroller. While this relationship is known in the art, it will also beapparent that if highly convex rollers are used, greater alignmentprecision is required to obtain consistent control of slot width.However as already noted, precise circumferential alignment of theforming rollers with each slot is difficult to achieve in a costeffective mechanized process.

It is therefore an additional purpose of the present invention toprovide a method to narrow the width of slots placed in the wall ofmetal tubulars by forming the slot edges and to additionally control theslot divergence rate or depth to which it is narrowed, thereby retainingseveral of the advantages enjoyed by forming methods in the prior artrelying on application of pressure along the slot edge while overcomingcertain shortcomings. This purpose is realized while practising themethod of the present invention by manipulating the forming tool shapeaccording to the following understandings. Without limiting finerdistinctions in geometry, the forming tool shape, in its region ofcontact with the work piece, may be generally characterized in terms ofits curvature in the longitudinal and transverse directions, whichdirections are with reference to cylindrical co-ordinates of the tubularwork piece. Curvature magnitude is to be understood as the inverse ofradius of curvature, and considered positive for convex forming toolshapes, zero for flat or straight shapes and therefore negative forconcave shapes. To obtain a greater divergence rate, the forming toolcurvature is decreased in one or both of the transverse and longitudinaldirections. Conversely to obtain a lessor divergence rate, curvature isincreased in one or both of the transverse and longitudinal directions.These curvatures are limited so that the curvature in the longitudinaldirection must not be significantly less than zero. The curvature in thetransverse direction must not be less than the tubular transversecurvature of the contacted surface. The tubular transverse curvaturesign is considered with respect to the forming tool reference; thus theouter surface transverse curvature sign is negative and the innerpositive.

Thus when the method of the present invention is used to form the edgesof longitudinally oriented slots, and it is desired to obtain slotshaving a high rate of divergence by increasing the forming toolcurvature in the transverse direction, the difficulty of alignmentexperienced by methods in the prior art relying on forming by applyingpressure along the slot edges is removed.

While slotted liners for wells are generally provided withlongitudinally oriented slots, other slot orientations may be desirablefor well completions or indeed for other applications such as filtersused for various fluid cleaning purposes. Methods in the prior art, asdescribed by Hruschak, are limited to longitudinally oriented slots.

A further purpose of the present invention is therefore to provide amethod to narrow the width of slots placed in the wall of tubulars atany orientation, where such slotted tubulars are suitable for use asscreens in wells or other similar filter applications. This purpose isrealized because the sweep pattern employed in the method of the presentinvention ensures that all the slot edges are traversed regardless oforientation. The sweep pattern may be adjusted to improve the efficiencyof the forming process, however a generally helical pattern ispreferred.

DESCRIPTION OF THE DRAWINGS

FIG. 1 Illustration of typical slotted liner tubular interval havingcircumferentially distributed longitudinal slots in rows.

FIG. 2 Illustration of the slots contained in the slotted linerillustrated in FIG. 1 being formed by a contoured forming roller.

FIG. 3 Cross-sectional view of a fixture carrying three radially opposedforming rollers, which assembly together comprises a forming head.

FIG. 4 Illustration of machine architecture employing rotating forminghead.

FIG. 5 Illustration of roller geometry parameters

FIG. 6 Plan view of longitudinal slot transversely rolled showing arealextent of plastic deformation zone.

FIG. 7 Cross-sectional view of slot shape after forming by transverserolling.

DESCRIPTION OF THE PREFERRED EMBODIMENT

According to the preferred embodiment of the present invention, a metaltubular 1, the work piece, is provided having an exterior surface 2 andinterior surface 3 and having one or more longitudinal slots 4, eachhaving exterior longitudinal peripheral edges 5 and 6 as illustrated inFIG. 1. To reduce the width between exterior peripheral edges 5 and 6 ofslots 4 a contoured rigid forming tool, configured as a forming roller 7in the preferred embodiment, is provided and forced into contact withthe exterior surface 2 of the metal tubular 1 to apply localizedpressure while being moved largely transversely with respect to thetubular pipe along a helical path 8 as shown in FIG. 2. Sufficientpressure must be applied through the contoured forming roller 7 toplastically deform the peripheral edges 5 and 6 of the slots 4 as theroller traverses the slots 4 following the helical path 8. The pitch 9and total length of the helical path 8 is adjusted to ensure thelocalized zones of plastic deformation caused when the rollersequentially traverses a given slot occur at close enough intervals toeffectively continuously deform the slot along its entire length.

FIG. 2 illustrates the forming process at an intermediate step where theslot width at peripheral edges 5 and 6 of slots already traversed by theforming roller 7 following the helical path 8 have been narrowed. Thelocation of Section A—A, shown in FIG. 2 was selected to show thecontrast in slot width between the longitudinal interval of slotsalready traversed and the remainder of the slot length yet to betraversed.

Given the teachings of the present method, it will be apparent to oneskilled in the art that for a given work piece there exists arelationship between the reduction in slot width and the:

-   -   radial force applied to the forming roller,    -   shape of the forming roller,    -   pitch of the helical forming path,    -   number of times the roller traverse is repeated, and    -   to a limited extent, the speed at which the roller is moved        relative to the tubular surface.

The manner in which these variables interact to control the degree offorming is highly interactive and is best determined empirically but maybe generally understood as follows:

-   -   The greater the available force the greater the amount of        plastic deformation possible.    -   For a given available force, the shape of the forming roller        generally controls the magnitude and longitudinal extent over        which the reduction in slot width occurs for a single traverse        of the roller over a slot. Manipulation of the roller shape is        generally constrained such that an increase in the longitudinal        extent of forming can only be obtained at the expense of slot        width reduction and vice versa.    -   The pitch of the helical forming path must be co-ordinated with        the axial extent over which the reduction in slot width occurs        for a single traverse of the roller over a slot to ensure the        width reduction occurs over the entire longitudinal extent of        the slot.    -   Repeated traverses of the roller over the same slot location at        the same load tend to increase the amount of deformation by        incrementally smaller amounts as the number of traverses is        increased.    -   Speed must not introduce undesirable dynamic effects.

While it is expected that for most applications a satisfactory reductionin slot width can be achieved with a constant roller load and helicalpitch, it will be evident that both these control parameters may bevaried during forming to increase or decrease the magnitude of slotnarrowing over specific axial intervals along the tubular length. Forexample, it may be necessary to decrease the pitch when the formingroller is traversing the end regions of slots to obtain a satisfactorydegree of narrowing.

For production purposes, it is generally desirable to obtain the maximumpitch as this increases the rate of forming for a given speed. As notedabove, the pitch, while influenced by other factors, is limited by themaximum allowable radial force.

The maximum radial force which may be applied to the forming roller is afunction of the manner in which the slotted tubular is supported andhence how the force applied through the roller is reacted. It will beevident that there exist numerous means of supporting the work piece andreacting the radial force applied through a forming roller 7 includingproviding support on the inside of the tubular. However it is mostconvenient if fixturing acting primarily on the exterior surface 2 cansupport the work piece and is arranged to react the radial force appliedthrough a forming roller to the work piece through one or more opposingradial rollers acting at or near the same axial plane. The rollers mostconveniently apply these opposing radial forces when mounted in a commonrigid frame, similar to the manner of a ‘steady rest’ commonly used tosupport a long work piece in a lathe. It will be evident that more thanone of these rollers can be arranged to act as forming rollers, in whichcase interleaved ‘multiple start’ helical paths can be generated as afunction of the pipe rotation with respect to the rollers withassociated benefits in production rate.

One such configuration found to be practical is shown in FIG. 3. Asillustrated there, the axles 10 of three radially opposed formingrollers 7 are attached to the pistons 11 of three hydraulic actuators12, each positioned at approximately 120° around the work piece andfastened to the forming head frame 13. Load is applied to the formingrollers 7 by application of fluid pressure 14. Together this assembly isreferred to as a forming head 15. This configuration substantiallyreduced the tendency of the work piece to bend and provides a radialload capacity enabling a reasonably large formed zone without permanentdistortion of the work piece cross sectional shape for typical slottedtubular materials.

Continuing consideration of the manner in which the work piece issupported, the means by which one or more forming rollers 7 carried in aforming head assembly 15 is caused to move in a helical path 8, withrespect to the work piece, may be accomplished in various ways. Howevertwo principal architectures present themselves as most practical.Firstly, with respect to the earth, the work piece may be rotated andthe forming head caused to move axially in synchronism with therotational position, in the manner of a lathe used for threading orturning operations. Secondly, the forming head may be rotated withrespect to the earth and the work piece caused to move axially throughthe head without rotation, in synchronism with the forming rollerrotation.

In its preferred embodiment, the present invention employs the second ofthese architectures in a machine illustrated in FIG. 4. As shown there,the work piece or slotted metal tubular 1 is positioned with respect tothe forming head 15 by guide rollers 16 and drive roller 17. Forceapplied by hydraulic actuators 18 ensure the work piece is held and thedrive roller 17 develops sufficient friction to axially displace thework piece with respect to the forming head 15 while the forming head isrotating. The forming head 15 is mounted in bearings 19 allowing it tobe rotated by means of a drive belt 20 driven by motor 21. Thecombination of axial and rotational motions thus provided, causes theforming rollers 7 to follow helical paths along the outside surface ofthe work piece, the pitch 9 of which helical paths is controlled byadjusting the axial feed rate with respect to the rotating speed of theforming head.

As introduced above, the shape of the forming tool, or preferablyforming roller, may be used in combination with the other processcontrol variables of load, pitch and number of roller traverses toadjust the amount by which a slot is narrowed and the depth over whichthe narrowing occurs. The means by which roller shape controls theseoutcomes may be generally characterized in terms of the roller radius(R) 22 and profile radius (c) 23 as illustrated in FIG. 5. While theprofile shape may take various forms, a simple convex shape, as shown inFIG. 5, was found to provide satisfactory control of slot widthreduction when forming longitudinal slots following a largely transversehelical path as anticipated for the preferred embodiment.

To understand how these geometric parameters may be advantageouslymanipulated, consider the shape of the zone of plasticity caused as aroller, having a generally smooth convex profile shape, crosses thecentre of a slot following a largely transverse path. As shown in FIG. 6the width of the areal extent of plastic deformation 24 as a function ofposition along the roller path 25, caused when the roller traverses theslot, tends to be greatest nearest the slot. This occurs because thestressed material is least confined at the slot and creates an effectiveformed length (z) 26 for a single traverse of the forming roller over aslot. Correspondingly, the depth of plastic deformation is greatest atthe slot, producing narrowing of the through wall channel shape toforming depth (d) 27 as shown in FIG. 7. It will be apparent that if thepitch exceeds z, the areal extent of successive roller traverses willnot overlap sufficiently along the slot edges to effectivelycontinuously narrow the slots over their entire length, and the slot issaid to be under-formed. Within the context of the preferred embodiment,there is a maximum allowable roller load (F) dependent on the structuralcapacity of the work piece when loaded by the forming rollers within theforming head. Furthermore the amount by which the slot width is to benarrowed (Δw) may be treated as a given for purposes of understandingchoice of forming roller radius (R) 22 and profile radius (c) 23. Tomaximise production rate it is preferable to produce the requiredreduction in slot width by only rolling the surface of the work pieceonce with the roller load at or near the maximum allowable. Under theseassumptions then, for a given roller radius 22, there exists a minimumprofile radius (c), referred to as the critical radius, for which thedesired Δw is obtained for a single traverse of the slot, as illustratedin FIG. 6, with corresponding value of formed length z. For these‘optimum’ conditions the pitch must largely correspond to z to avoideither under or over forming the slot. Pitch (P) may therefore betreated as a dependent variable. Such a minimum profile radius is alsooptimised to form the edges most completely to the ends of the slots.

Next consider the effect of variations in R assuming c is ‘optimally’selected as just described. It will be apparent that as R is decreasedthe extent of the zone of stress under the roller is reduced in thedirection of rolling (normal to the slot direction) therefore c must beincreased to maintain the condition of constant Δw and z willcorrespondingly increase. Because pitch increases with z the rate ofproduction increases for decreasing R. It should also be apparent thatthe forming depth (d) 24 will decrease as R is decreased due to thereduced extent of the zone of stress under the roller, normal to theslot direction. This provides a means to control the shape of the formededges concurrent with the rate of divergence in the flow channel.

However, it is preferable if the profile radius (c) is somewhat greaterthan the critical value as this allows greater flexibility inaccommodating randomness in the numerous variables, such as materialproperties, affecting slot width. The greater flexibility derives fromthe fact that as c becomes greater than critical, the pitch must onaverage be reduced to maintain Δw constant. Thus if variations inparameters such as an decrease in strength require less forming, thepitch may be increased to compensate without causing under forming. Thisability to use variation in pitch to provide fine control of the finalslot width is of practical benefit for automating the process. Inparticular, if the slot width is measured directly after the slots areformed, variations from the desired width may be compensated forsubsequent formed intervals by adjusting either the load or pitch butpreferably the pitch. This feedback task may be performed manually orautomated using a suitable means to measure slot width.

Therefore in its preferred embodiment, the roller and profile radii areselected to ensure adequate sensitivity of slot width to pitch ismaintained to facilitate process control without compromising theability of the roller to form the edges of slots near their ends.

1. A method of reducing slot width in slotted tubular liners, comprisingthe steps of: (a) providing a slotted tubular liner having an interiorsurface, an exterior surface and a plurality of slots extending betweenthe interior surface and the exterior surface; (b) providing at leastone contoured rigid forming tool; (c) applying pressure to a selectedone of the interior surface and the exterior surface of the slottedtubular liner with the at least one contoured rigid forming tool; and(d) moving the at least one contoured rigid forming tool, relative tothe slotted tubular liner, in a sweep pattern traversing the selectedone of the interior surface and the exterior surface of the slottedtubular liner, until plastic deformation narrows the width of theplurality of slots, as measured at the selected one of said interior andexterior surfaces, to within desired tolerances: wherein the directionof relative movement of the at least one rigid forming tool is largelytransverse to the longitudinal axis of the slotted tubular liner.
 2. Themethod as defined in claim 1, the contoured rigid forming tool being aroller.
 3. The method as defined in claim 1, there being severalcontoured rigid forming tools positioned at spaced intervalscircumferentially in relation to the selected one of the exteriorsurface and the interior surface of the slotted tubular liner.
 4. Themethod as defined in claim 3, there being three contoured rigid formingtools positioned at 120 degree spaced intervals.
 5. The method asdefined in claim 1, the sweep pattern including two or more sweep paths,the sweep paths being closely spaced with overlapping zones of localizedplastic deformation in proximity with edges of each slot.
 6. The methodas defined in claim 1, the sweep pattern being a helical path.
 7. Themethod as defined in claim 1, the slotted tubular liner being rotatedand the at least one contoured rigid forming tool being non-rotatablyfixed and moving axially along the slotted tubular liner.
 8. The methodas defined in claim 1, the at least one contoured rigid forming toolbeing rotated and the slotted tubular liner being non-rotatably fixedand moving axially past the at least one contoured rigid forming tool.9. The method as defined in claim 1, the at least one contoured rigidforming tool being rotated and also being moved axially along theslotted tubular liner, with the tubular liner being non-rotatably fixedand axially stationary.
 10. The method as defined in claim 1, the atleast one contoured rigid forming tool being non-rotatably fixed, withthe tubular liner being rotating and moving axially past the at leastone contoured rigid forming tool.
 11. The method of claim 2 wherein theroller is contoured to have a convexly radiused profile.
 12. The methodof claim 2 wherein the roller is contoured to include a cylindricalportion.
 13. Apparatus for reducing the width of slots in a slottedround tubular liner having an interior surface, an exterior surface anda plurality of slots extending between the interior surface and theexterior surface, said apparatus comprising: (a) a forming head framehaving a centroidal axis, said forming head frame being adapted toreceive a slotted liner such that the longitudinal axis of the slottedliner substantially coincides with the centroidal axis of the forminghead frame; (b) one or more forming tools; (c) mounting means, formounting said one or more forming tools to the forming head frame suchthat each of the one or more forming tools may be extended radiallyinward relative to the centroidal axis of the forming head frame; (d)actuating means, for urging the one or more forming tools into contactwith the exterior surface of the slotted liner under a selectivelyvariable radial load; (e) rotation means, for causing rotationalmovement of the one or more forming tools relative to the axis of theslotted liner; and (f) axial movement means, for causing axial movementof the forming head frame relative to the slotted liner; whereinconcurrent actuation of the rotation means and the axial movement means,with the one or more forming tools in contact with the slotted linerunder radial loading of sufficient magnitude, will cause each formingtool to trace a sweep pattern around the exterior surface of the slottedliner, until plastic deformation narrows the width of the slots, asmeasured at the exterior surface of the liner, to within desiredtolerances.
 14. The apparatus of claim 13 wherein the sweep pattern is ahelical path.
 15. The apparatus of claim 13 wherein, with respect to atleast one of said one or more forming tools: (a) the mounting meanscomprises a hydraulic actuator mounted to the forming head frame, saidactuator having a piston extensible radially inward and retractableradially outward relative to the centroidal axis of the forming headframe; (b) the forming tool is mounted to the radially inward end of thepiston; and (c) the actuating means comprises means for deliveringhydraulic pressure to the actuator, for selectively extending orretracting the piston.
 16. The apparatus of claim 13 wherein: (a) theforming head frame is axially stationary; (b) the rotation meanscomprises means for rotating the forming head frame about its centroidalaxis; and (b) the axial movement means comprises means for moving theslotted liner axially through the forming head frame.
 17. The apparatusof claim 13 wherein: (a) the forming head frame is non-rotatably fixed;(b) the rotation means comprises means for rotating the slotted linerabout its centroidal axis; and (c) the axial movement means comprisesmeans for moving the forming head frame axially along the slotted linersuch that the liner moves through the forming head frame.
 18. Theapparatus of claim 13 wherein: (a) the slotted liner is non-rotatablyfixed; (b) the rotation means comprises means for rotating the forminghead frame about its centroidal axis; and (c) the axial movement meanscomprises means for moving the forming head frame axially along theslotted liner such that the liner moves through the forming head frame.19. The apparatus of claim 13 wherein: (a) the forming head frame isnon-rotatably fixed; (b) the rotation means comprises means for rotatingthe slotted liner about its centroidal axis; and (c) the axial movementmeans comprises means for moving the slotted liner axially through theforming head frame.
 20. The apparatus of claim 13 wherein at least oneof the one or more forming tools is a roller.
 21. The apparatus of claim20 wherein the roller is contoured to have a convexly radiused profile.22. The apparatus of claim 20 wherein the roller is contoured to includea cylindrical portion.
 23. The apparatus of claim 20 wherein the rolleris rotatable around an axis substantially parallel to the centroidalaxis of the forming head frame.
 24. The apparatus of claim 20 whereinthe roller is a castering roller.
 25. The apparatus of claim 13, furthercomprising rotational speed control means, for regulating the rotationalspeed of the one or more forming tools relative to the slotted liner.26. The apparatus of claim 13, further comprising axial speed controlmeans, for regulating the axial travel speed of the forming head framerelative to the slotted liner.
 27. The apparatus of claim 16 wherein therotation means comprises a plurality of bearings disposed about theexterior circumferential surface of the forming head frame, saidbearings being adapted to support and retain the forming head frame andallow it to rotate about its centroidal axis.
 28. The apparatus of claim27, further comprising drive means for rotating the forming head frame.29. The apparatus of claim 28 wherein the drive means comprises a motor.30. The apparatus of claim 29, further comprising a drive belt fortransferring rotational motion from the motor to the forming head frame.31. The apparatus of claim 16 wherein the axial movement meanscomprises: (a) a plurality of rollers, said guide rollers being adaptedto engage the slotted liner so as to keep the liner in substantiallyco-axial alignment with the forming head frame, at least two of saidguide rollers being disposed so as to support the weight of the liner;and (b) at least one drive roller, said drive roller being rotatable bya power source about an axis transverse to the axis of the liner; and(c) drive roller engagement means, for engaging the at least one driveroller with the liner such that rotation of the at least one driveroller will cause axial movement of the liner.
 32. The apparatus ofclaim 31 wherein the drive roller engagement means comprises a hydraulicactuator for urging the at least one drive roller into contact with theliner.
 33. The apparatus of claim 13 having a plurality of forming toolspositioned at spaced intervals circumferentially in relation to theexterior surface of the slotted tubular liner.